1. Field of the Invention
The present invention relates to a method for manufacturing a group III nitride semiconductor device.
2. Prior Art
Extensive researches is now underway on nitride-gallium and related compound semiconductor as a material system for short wavelength light emitting devices, in particular short wave length semiconductors lasers. A semiconductor laser device is manufactured by forming a semiconductor single-crystal film on a crystal substrate. Characteristics of a light emitting device as a laser device are greatly dependent on the density of crystal defects in the single-crystal film. From this point of view, the substrate is preferable to be made of a material having the physical-property constants such as crystal structure, lattice constant, and thermal-expansion coefficient the same as those of a semiconductor crystal to be expitaxially grown on the substrate. For example, it is ideal to use a single crystal of gallium nitride (hereinafter referred to as GaN), which is the same as an epitaxial film. However, it is very difficult to form a large crystal of GaN. A GaN single-crystal ingot having a size usable as a wafer has never been formed. Therefore, a sapphire substrate has been used as a substrate.
Metal-organic chemical vapor deposition (hereinafter referred to as MOCVD) is most generally used to form a GaN single-crystal film. In this method, trimethylgallium (referred to as TMG hereinafter) is used as a group III precursor material, and ammonia (referred to as NH3 hereinafter) is used as a group V precursor material for the material of the MOCVD.
When forming a GaN semiconductor film by the above method, the crystal is grown at the temperatures of 900-1100xc2x0 C. At the higher temperature, semiconductor crystal such as gallium arsenide (GaAs) is unstable so that it is not a proper material for a substrate. Thus, single-crystal sapphire is adopted as a substrate. This is because sapphire is stable at the above high temperature though the physical properties of sapphire are different from those of GaN.
However, the direct growth of GaN on a sapphire substrate can not produce a higher-quality single-crystal film for a light emitting element, because sapphire has a lattice constant different from that of GaN by approximately 14%. Therefore, an approach, known as the two-step-growth method, to moderate the misfit of the lattice constant between a GaN single-crystal film and sapphire, has been developed. The steps in the two-step-growth method are: forming a low temperature buffer layer of aluminum nitride (AlN) or GaN on a sapphire substrate at a low temperature of 400-600xc2x0 C., heating the substrate with the buffer layer, and then epitaxial-growing GaN on the buffer layer. Thus, the two-step-growth method has stably produced a comparatively smooth single-crystal film.
However, it has been found that the GaN film produced by the above method has high-density defects of 1xc3x97108-1xc3x971010 (1/cm2), and the above value shows a very high density, which is 10,000-100,000 times greater than that of GaAs. Such a defect is called xe2x80x9cthreading dislocationxe2x80x9d, which is linearly extending crystal defect that penetrates a crystal film from its top to the bottom. The threading dislocation works as the non-radiative recombination center of carriers. Therefore, it is known that as the threading dislocation density increases, the light emitting efficiency of the semiconductor crystal decreases.
Though a light emitting efficiency characteristic of GaN is less affected by the dislocation density, compared to that of a conventional optical semiconductor such as GaAs, the existence of a large number of dislocation degrades the light emitting efficiency.
The dislocation density depends on conditions of the epitaxial growth, and particularly depends on the growth pressure. Moreover, it is known that the threading dislocation density decreases as the growth pressure increases, as described in Japan Journal of Applied Physics, Vol. 37 (1998) p. 4460-4466. Therefore, the epitaxial growth on a sapphire substrate is generally performed at approximately atmosphere pressure (760 torr).
It is known that crystal nuclei grow three-dimensionally, like islands, at the beginning of growth, and that they then combine and grow two-dimensionally, when a low-temperature buffer layer of GaN or AlN is formed on a sapphire substrate and then the GaN crystal is grown on the buffer layer at a higher temperature (approximately 1000xc2x0 C.). Therefore, a hexagonal-pyramidal hole called a xe2x80x9cpitxe2x80x9d may be formed in the GaN expitaxial film. The presence of pits makes the production of a good device on or near the pit difficult. That is, a higher pit density of an expitaxial wafer may result in the decrease of device production during manufacturing.
It is known that the pit density also greatly depends on the growth pressure and that it increases more as the growth pressure increases. Therefore, as described above, it is clear that a dislocation density and a pit density are mutually contradictory. Thus, a problem arises that the increase of the growth pressure for decreasing a dislocation density may cause the increase of a pit density, thereby leading to the decrease of a manufacturing yield of a device.
An object of the present invention is to solve the above problems to provide a nitride semiconductor device and a method for manufacturing the nitride semiconductor device which has a reduced dislocation density while suppressing the generation of pits in a GaN layer.
A method for manufacturing a nitride semiconductor device according to the present invention is characterized in that the method comprises the steps of successively depositing a group III nitride semiconductor (InxGa(1xe2x88x92xxe2x88x92y)AlyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61)) on a flat sapphire substrate by metal-organic chemical vapor deposition. The method comprises the steps of: forming a low temperature buffer layer on the sapphire substrate, forming a first gallium nitride layer on the low temperature buffer layer by supplying a first mixed gas containing no dopant component, and forming a second gallium nitride layer containing a dopant on the first gallium nitride layer while filling voids and flattening a surface of the first gallium nitride layer by supplying a second mixed gas containing a dopant component.
A nitride semiconductor device according to the present invention is characterized in that the device is manufactured by depositing a group III nitride semiconductor (InxGa(1xe2x88x92xxe2x88x92y)AlyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61)) on a flat sapphire substrate. The group III nitride semiconductor device comprises a low temperature buffer layer deposited on the sapphire substrate; a first gallium nitride layer having a first predetermined thickness, containing no dopant component, and being deposited on the low temperature buffer layer; and a second gallium nitride layer having a second predetermined thickness containing a dopant component, the second gallium nitride layer being deposited on the first gallium nitride layer; wherein the second gallium nitride layer is deposited while flattening the surface of the first gallium nitride layer by filling voids in the first gallium nitride layer.